In an access network for transferring data between a base station of a carrier and a user site, photonic networks with large capacity and high speed have been increasing. Wavelength Division Multiplex-Passive Optical Networking (WDM-PON), a next generation optical access network system that assigns an individual wavelength to each user using a Wavelength Division Multiplex (WDM) system, has been reviewed for replacement of a conventional Time Division Multiplex (TDM) system. In the WDM-PON system, the following network configuration is proposed. The configuration is that a wide band light source of continuous wave (CW) is provided for a base station device and a Reflective Semiconductor Optical Amplifier (RSOA) is provided for a user site device, and the user site amplifies and modulates a CW signal light transmitted from the base station and returns the CW signal light.
As an example, FIG. 1 illustrates such an optical access system. In the optical access system, an optical line terminal (OLT) 310 that is a base station device and an optical network unit (ONU) 320 that is a user site device are coupled through an Arrayed Waveguide Grating (AWG) 331 by a single mode optical fiber 330. The AWG 331 distributes signal lights of a plurality of wavelengths (wavelength 1, wavelength 2, wavelength 3, . . . wavelength N) for each wavelength to the ONU 320 of each user site. In the OLT 310, an upstream signal CW light source 311 and an upstream signal receiver 312 are coupled to an optical circulator 313 and such a set of the coupled elements is coupled to an AWG 314 for each of the wavelengths. Signal lights emitted from the upstream signal CW light source 311 in the OLT 310 are transmitted from the OLT 310 through the optical circulator 313 and the AWG 314. The signal lights are input to the AWG 331 through the optical fiber 330 and are input to the ONU 320 after the AWG 331 divides the signal lights for each of the wavelengths of the respective user site.
The ONU 320 includes a reflective semiconductor optical amplifier 321 and the reflective semiconductor optical amplifier 321 amplifies and returns the input signal light. By controlling a drive current or a voltage in the reflective semiconductor optical amplifier 321, the returned signal lights are modulated by data pattern. The returned upstream signal lights are transmitted to the OLT 310 again through the AWG 331 and the optical fiber 330. In the OLT 310, the returned upstream signal lights that are input to the signal receiver 312 through the AWG 314 and the optical circulator 313 are demodulated.
In the above-described optical access system, respective user sites are provided with the ONU 320. In the respective ONUs 320, the reflective semiconductor optical amplifier 321 is provided.
It is preferable that the reflective semiconductor optical amplifier 321 may be manufactured with a low cost and may be operated at uniform input light intensities for various wavelengths.